Photosynthesis

and

CellularRespiration

Moly-Mod Lab

Tahoma Jr. High8th Grade ScienceMaple Valley, WA

We learned howhydrocarbon fuelsare put togetherand have energythat can be released.

But we can’t eat oil,or breathe in methaneand break it downfor energy.

Instead we use a similar chemical: carbohydrates.

But how are they made?

Oil comes from old partially decomposed plants, so let’s “be a plant” for a while.

What do plants need every few days or they die?

WATER! – make 6 H2O molecules(use the short bonds AND set them aside)

They also need a carbon source to put sugar together. What common gas has carbon in it?

That’s right! – CO2.Make 6 CO2 molecules

Now we need an energy source that will break the bonds to allow the atoms to re-arrange themselves into a sugar molecule. What is it?

Yup… the Sun !!

The sunlight will release the carbon to make a “skeleton” for the glucose (sugar) molecule. Break

your six CO2 molecules apart and remove the carbon on each of them. What’s left without the carbon?

O2 – put them back together

(technically, this isn’t what happens, but we’ll keep things simple in this model)

You know plants give off nice, clean, fresh oxygen – but it breaks things down, so is a waste gas to plants during photosynthesis. (Later they will need some to break down their own food).

But for now, let’s put the O2 molecules off to the side and forget about them – they are waste. DO NOT TOUCH THEM UNTIL TOLD TO !!

As an FYI – since we’re dealing with molecules

now, here’s a CHLOROPHYLL molecule:

Now we can make the glucose (sugar) molecule. We’ve got 6 carbons, 6 water molecules, light and chlorophyll to work with. There are many combinations possible, so we’ll help you by giving you the carbon skeleton layout.

CC

C

CC

C

Oall bondsnow are singleand it doesn’t matter ifthey are short or long

From there, there’s only ONE way to finish the molecule without having any bond holes open or bonds hanging out in space.

Finish a glucose molecule…(hint: don’t have two oxygens bonded to each other)

Have your teacher check when you think you’re done. You should not have any atoms left over

(except for the six O2 molecules sitting on the side that were waste gas)

Count the atoms – what is the formula for glucose?

C6H12O6

that’s the formula for glucose

sugar is a “carbohydrate”:

carbon, hydrogen, lots of oxygen

GLUCOSE IS THEN CONVERTED TO

STARCH FOR LONG-TERM STORAGE

Photosynthesis is probably the MOST important chemical reaction on Earth !!

It take sunlight energy (which can’t be stored) and turns it into storable chemical

energy (glucose).

HERE’S A FAMILY PORTRAIT OF WORK

GROUPS WHO COULD NOT PUT THEIR GLUCOSE TOGETHER FAST

ENOUGH

extinctfossilized plants

get it ?

NOW WE’VE MADE A GLUCOSE MOLECULE.

SO WHAT ARE WE GOING TO DO WITH IT ?

C6H12O6

You are now going to “be an animal” for a while – but you can’t make your own food. You

are a “consumer” – not a “producer”.

So, what do animals need to break down the food they eat for energy?

That’s right! – O2. Get those oxygens back(you breathe them in) and then break down your

glucose molecule to individual atoms.

Now think about what you give off as waste after breaking down your food. If you breathe onto a mirror you will see one of the chemicals. It is…?

Yup… water! H2O (make 6 H2O molecules)

That’s right! – CO2. Bond those six carbons together with the 12 oxygens you have left. You should get 6 CO2 molecules – and you’re back

where you started from when you started as a plant!

Now, what gas are you breathing out (it’s the “opposite” of oxygen)?

X 6 6 X

You have just done Cellular Respiration and released its wastes in the process of breathing

CELLULAR RESPIRATION OCCURSIN CELL ORGANELLES (structures) CALLED

MITOCHONDRIA(you should have learned about these in 7th Grade)

Animals use oxygen (brought to the cells by red blood cells in the blood) to break down glucose (also brought by blood)…

and then release carbon dioxide and water.

This diagram shows 6 extra waters – but they simply go in and come right back out again – so many diagrams/equations don’t have them included.

OXIDIZING HYDROCARBONS (alkanes) ANDCARBOHYDRATES RELEASES SIMILAR PRODUCTS

BUT WITH PHOTOSYNTHESIS AND RESPIRATIONTHERE IS A TIDY COMPLETE RECYCLING OF MATTER

immediate chemical energyto do work

stored chemical energy

“free” light energy to trap

Now… although plants do PHOTOSYNTHESIS and animals do RESPIRATION, who did the plants make their sugar for,

and how do they get their energy for life?

The plants don’t make the sugar for the animals(most of the time at least - nectar and fruit are “bribes” to

pollinate and carry seeds away).

Plants make the sugar for THEMSELVES – and therefore, they must also do RESPIRATION.

Don’t forget this when you see diagrams. Plants do both photosynthesis and cellular respiration, but the important one is photosynthesis to make the sugar in the first place. Animals came along later and found a way to exploit this

chemical energy source by eating plants.

YOUR TEACHER WILL NOW LEAD A CLASS DEBRIEF ON PHOTOSYNTHESIS AND RESPIRATION EQUATIONS

COUNT THE ATOMS AND BONDS CAREFULLY AND PUT THEM IN THE BAG ALONG WITH THE PAPER LIST

MAKE SURE NOTHING IS MISSING !!

On the Chemistry Part B Test you should know how to write a balanced chemical equation for:

photosynthesis

and

cellular respiration

end show